The main difference between a leech and other critters considered bloodsuckers is species. Leeches are unique because they are worms that live in wet, watery areas. Leeches are creepy, yet fascinating, because they release a toxin into their prey that acts as a numbing agent, allowing them to feed without being discovered. It's hard to escape leeches because they have two suckers on each end of their bodies.
Sometimes leeches are referred to as bloodsuckers; this is a general term applied to any species that sucks the blood of people and animals. For instance, bedbugs and vampire bats are also referred to as bloodsuckers. Having one's blood sucked by a leech while swimming is no fun. However, throughout history, leeches have been used as a natural medical cure for various human ailments.
The practice of bloodletting has been used by ancient Greeks and medieval Europeans. Medical practitioners of old applied leeches all over a patient's body to release toxins thought to be in diseased blood. Surprisingly, leeches continue to be used in modern medicine. Most notably, leeches are used as a natural anticoagulant during skin graft procedures on burn victims. The leeches remove pools of blood from underneath the skin, preventing clotting.